The present invention relates generally to a multilayered printed wiring board capable of realizing high-density packaging and a method for manufacturing the same, and particularly to a printed wiring board having an odd number of conductive layers, a printed wiring board having build-up layers formed by using the additive method and the like, a method of forming interconnecting through holes for electrically connecting conductive layers and narrowing the pitch between solder balls for external connection and the interconnecting through holes.
Conventional printed wiring boards include those having conductive layers 911 to 914 built up successively, as shown in FIG. 42. The conductive layers 911 to 914 are electrically connected to one another via interconnecting through holes 931 to 933. Insulating layers 921 to 923 are interposed between the conductive layers 911 to 914, respectively.
The conductive layer 911 is a component-connecting layer on which an electronic component 961 is mounted and conducts electric currents in and out of the electronic component 961. The conductive layer 911 which is one of the outermost layers and the electronic component 961 are electrically connected to each other by bonding wires 962. The conductive layer 914 which is the other outermost layer serves as an external connecting layer for connecting external connecting terminals 97 and leading electric currents in and out of a printed wiring board 941. The internal conductive layers 912 and 913 are electric current transmitting layers for transmitting internal currents of the printed wiring board 941.
Next, the method of manufacturing the above printed wiring board will be described.
First, as shown in FIG. 43, conductive layers 912 and 913 are formed on the upper side and lower side of an insulating layer 922 respectively. Further, interconnecting through holes 932 are formed through the insulating layer 922, and the wall of each interconnecting through hole 932 is covered with a metal plating film 95. A resin 92 is then packed in the interconnecting through holes 932.
Next, an insulating layer 921 and a copper foil are laminated on the upper side of the insulating layer 922, while an insulating layer 923 and a copper foil are laminated on the lower side, followed by etching of the copper foils to form conductive layers 911 and 914.
Subsequently, as shown in FIG. 44, interconnecting through holes 931 and 933 are formed through the insulating layers 921 and 923 to expose the surfaces of the internal conductive layers 912 and 913, respectively.
Then, as shown in FIG. 42, a metal plating film 95 is formed on the walls of the interconnecting through holes 931 and 933, and external connecting terminals 97 are bonded onto the surface of the outermost conductive layer 914.
Thus, the printed wiring board 941 can be obtained.
By repeating the procedures shown in FIGS. 43 and 44, the number of conductive layers to be built up in the printed wiring board 941 can be increased. The thus obtained printed wiring board has insulating layers and conductive layers built up alternately both on the upper side and on the lower side of the center insulating layer 922. Therefore, an even number of conductive layers are formed according to the above method.
However, the conventional method of manufacturing printed wiring boards as described above is not suitable for building up an odd number of conductive layers, although it can build up an even number of conductive layers efficiently.
To describe, for example, a case where a printed wiring board having five conductive layers 910 to 914 built up, as shown in FIG. 45, is manufactured, the second to fifth conductive layers 911 to 914 are built up first, as shown in FIG. 46, in the same manner as described above, except that the conductive layer 914 is an unpatterned copper foil.
Next, as shown in FIG. 47, the conductive layer 914 is removed completely, and then interconnecting through holes 931 are formed, as shown in FIG. 48, followed by formation of a metal plating film 95 on the wall of each through hole 931. Subsequently, as shown in FIG. 49, prepregs are laminated and press-bonded to form insulating layers 920 and 924. Conductive layers 910 and 941 are then formed on the surfaces of the insulating layers 920 and 924 respectively, followed by formation of interconnecting through holes 930 and 933 through the insulating layers 920 and 924 respectively, as shown in FIG. 50. A metal plating film 95 is formed on the walls of the through holes 930 and 933, as shown in FIG. 45.
As described above, when a printed wiring board having an odd number of conductive layers is manufactured, it is necessary, in order to prevent warping of the press-bonded printed wiring board from occurring, to carry out, after formation of the internal conductive layers 911 and 914, the procedure of removing the conductive layer 914. Thus, the conventional method requires wasteful a procedure and is an extremely inefficient manufacturing method. Further, the insulating layers formed are too thick to meet the purpose of achieving downsizing of printed wiring boards.
Under such circumstances, it can be considered to form an insulating layer 920 and a conductive layer 910 only on one side of the insulating layer 921. In this case, however, warping of the printed wiring board can occur in the step of press-bonding a prepreg for forming the insulating layer 920.
Meanwhile, in a multilayer build-up type printed wiring board, the internal insulating layers 921 and 923 to be embedded in it are resins, so that they have high coefficients of water absorption of 0.5 to 1.0% and have high water contents. The water is vaporized naturally with passage of time to assume the form of water vapor which collects mainly, for example, between the insulating layer 921 and the adjacent insulating layers 922 and 920 and between the insulating layer 923 and the adjacent insulating layers 922 and 924.
Accordingly, it is likely that the interlayer adhesion is lowered and that the layers undergo delamination. Particularly, the greater the number of layers laminated, the greater becomes the number of water-containing internal insulating layers, and the higher becomes the tendency of interlayer delamination.
Meanwhile, referring to manufacturing of printed wiring boards, there is a method invented by us previously and disclosed in Japanese Patent Application No. Hei 8-21975. That is, as shown in FIG. 51, a conductive layer is formed on each insulating layer in step S91, and then interconnecting through hole-forming through holes are defined in each insulating layer in step S92. Steps S91 and S92 are repeated corresponding to the number n of insulating layers to be laminated. Next, in step S93, the number n of insulating layers are laminated via an adhesive material and positioned such that the through holes in the respective layers may communicate with one another to constitute interconnecting through holes. In step S94, the adhesive material is melted by heating and the like, and the layers are press-bonded together to form a multilayer substrate. In step S95, a conductive material, such as a solder and the like is packed into the interconnecting through holes to impart conductivity to them. Thus, a printed wiring board is obtained.
However, in the conventional method of manufacturing printed wiring boards described above, interconnecting through hole-forming through holes must be defined in each insulating layer independently. Accordingly, the method requires intricate procedures of defining through holes. Further, the through holes must be positioned. Particularly, with the reduction in the size of the interconnecting through holes, it is becoming difficult to carry out accurate registration of the through holes.
Meanwhile, in a multilayer printed wiring board, pads for connecting external terminals such as solder balls are provided on the outermost layer. In this case, the interconnecting through holes must be electrically connected with the pads by connecting circuits. However, the connecting circuits which occupy a large surface area are a hindrance in achieving high-density packaging on the substrate surface. Particularly, in a multilayer printed wiring board, it is necessary to form high-density wiring on the uppermost surface. Further, large amounts of electric too currents must fed in and out through the external connecting terminals.
The present invention is directed, in view of the problems inherent in the prior art described above, to provide a printed wiring board which can improve electrical properties of multilayered wiring boards and a method for manufacturing the same. Particularly, it is a first objective of the present invention to build up an odd number of conductive layers efficiently with no warping. A second objective of the present invention is to prevent delamination of layers. A third objective of the present invention is to form interconnecting through holes at accurate positions. A fourth objective of the present invention is to carry out transference of a huge amount of electrical information through solder balls for external connection and also to achieve high densification of surface packaging.
A first aspect of the present invention is a printed wiring board having an odd number n of conductive layers which are built up via insulating layers respectively and which are electrically connected to one another by interconnecting through holes, characterized in that the first conductive layer is a component-connecting layer on which an electronic component is to be mounted and which leads electric currents in and out of the electronic component. The n-th conductive layer is an external connecting layer for connecting external connecting terminals for leading currents in and out of the printed wiring board. The second to (nxe2x88x921)-th conductive layers are current transmitting layers for transmitting internal currents of the printed wiring board, and the surface of the n-th conductive layer is covered with the n-th and outermost insulating layer which is the outermost layer with the external connecting terminals being exposed.
What is noticeable most in the first aspect of the invention is that the printed wiring board has an odd number n of conductive layers and the surface of the n-th conductive layer is covered with the n-th and outermost insulating layer with the external connecting terminals being exposed.
In the first aspect of the invention, the odd number n means an integer excluding 1, which cannot be divided by 2 into a numeral with no decimal fraction, for example, 3, 5 and 7. The reason why 1 is excluded from the odd number n is that such a constitution having only one conductive layer cannot constitute a printed wiring board.
Actions and effects of the first aspect of the invention will be described.
The printed wiring board according to the first aspect of the invention has an odd number n of conductive layers formed between an odd number n of insulating layers respectively. The (n+1)/2-th insulating layer is a central insulating layer and has on the upper side and lower side the same number of insulating layers respectively. Accordingly, no warping occurs in the printed wiring board during press-bonding of prepregs for forming insulating layers.
Further, conductive layers can be built up on the upper side and lower side of the central insulating layer efficiently.
Therefore, the printed wiring board according to the first aspect of the invention is of the structure which facilitates building up of an odd number n of conductive layers.
Further, the n-th and last conductive layer is covered with the n-th and outermost insulating layer serving as the outermost layer. Accordingly, the n-th conductive layer is embedded in the printed wiring board. However, the external connecting terminals connected to the n-th conductive layer are exposed through connecting holes of the n-th insulating layer, so that electric currents can be led in and out of the printed wiring board through the external connecting terminals.
The external connecting terminals are preferably solder balls. The solder balls can stably lead electric currents in and out through the n-th conductive layer.
It is also possible to connect external connecting terminals to the surface of the n-th conductive layer and to build up an (n+1)-th conductive layer on the surface of the n-th insulating layer present on the n-th conductive layer. In this case, the resulting printed wiring board comes to have an even number of conductive layers. External connecting terminals can be connected to the surface of the (n+1)-th conductive layer.
The method of manufacturing the above printed wiring board can be exemplified as follows: a method of manufacturing a printed wiring board having an odd number n of conductive layers which are built up via insulating layers respectively and are electrically connected to one another via interconnecting through holes. The method comprising the steps of: interposing insulating layers between second to n-th conductive layers respectively and also forming interconnecting through holes for electrically connecting the conductive layers to one another; laminating a prepreg and a copper foil on the surface of the second conductive layer, while laminating and press-bonding a prepreg on the surface of the n-th conductive layer to form a multilayer substrate having an odd number n of insulating layers and also locating the second to n-th conductive layers as internal layers of the multilayer substrate; etching the copper foil to form a first conductive layer; forming interconnecting through holes and connecting holes in the first insulting layer and in the n-th insulating layer respectively; forming a metal plating film for electrically connecting the first conductive layer with the second conductive layer on the walls of the interconnecting through holes of the first insulating layer; and connecting external connecting terminals to the surface of the n-th conductive layer exposed through the interconnecting through holes of the n-th insulating layer.
What is most noticeable in this method is that a prepreg and a copper foil for forming the first conductive layer are laminated on the surface of the second conductive layer and that only a prepreg is laminated on the surface of the n-th conductive layer. When the prepregs and the copper foil are press-bonded, the first insulating layer and the n-th insulating layer are formed simultaneously by this press-bonding. Accordingly, the second to (nxe2x88x921)-th insulating layers already laminated into a single body receive, on the upper sides and the lower sides, thermal stress evenly from the prepregs during the press-bonding, so that no warping occurs in the printed wiring board.
Further, the n-th and last conductive layer is covered on the surface with an insulating layer formed by laminating and press-bonding a prepreg. In this state, no electric current can be led in and out through the n-th conductive layer. However, connecting holes are defined in the outermost insulating layer to expose the external connecting terminals through these connecting holes, and thus electric currents can be led in and out through the n-th and last conductive layer.
In addition, the external connecting terminals are preferably solder balls. The solder balls can lead stably electric currents in and out through the n-th conductive layer.
The conductive layers referred to above mean all sorts of conductive patterns which can be formed on the surfaces of insulating substrates, for example, wiring circuits, pads, terminals and lands. Conductive patterns are formed, for example, by etching metal foils or by metal plating.
The insulating layers include synthetic resin single substances, prepregs, etc. The synthetic resins include, for example, epoxy resins, phenol resins, polyimide resins, polybutadiene resins and fluororesins.
Further, the printed wiring board according to the first aspect of the invention can be utilized, for example, as memory modules, multichip modules, mother boards, daughter boards and plastic packages.
Methods of defining interconnecting through holes and connecting holes include, for example, irradiation of laser beams onto the insulating layers at the positions where holes are to be formed; chemical melting of the insulating layer at the positions where holes are to be formed; and machining using a drill.
A second aspect of the present invention is a printed wiring board comprising an internal insulating substrate having a conductor circuit formed on the surface, at least one internal insulating layer laminated on the surface of the internal insulating substrate, and an external insulating layer laminated on the surface of the internal insulating layer, the internal insulating layer and the external insulating layer having an internal conductor circuit and an external conductor circuit respectively; wherein the internal insulating layer is of a glass cloth-reinforced prepreg; and the external insulating layer is of a resin.
The glass cloth-reinforced prepreg referred to above means a material obtained by impregnating a glass cloth base material with a resin. However, in the second aspect of the invention, it is particularly preferred to use a prepreg containing 30 to 70% by weight of glass cloth. Thus, the coefficient of water absorption can be lowered to prevent interlayer delamination from occurring. Meanwhile, those prepregs which contain less than 30% by weight of glass cloth come to have high coefficient of water absorption to be liable to undergo interlayer delamination, whereas those which contain more than 70% by weight of glass cloth is likely to show low interlayer adhesion, since the absolute amount of resin is small.
Further, the outermost insulating layer may be formed using the same prepreg as used for the internal insulating layers.
It should be noted that in the printed wiring board according to the second aspect of the invention, interconnecting through holes, blind via holes, via holes, etc. can be formed in the internal insulating substrate, internal insulating layer(s) and external insulating layer. Further, on the external insulating layer, lands for mounting solder balls, a solder resist for securing insulation between external conductor circuits, etc. can be formed. That is, the printed wiring board according to the second aspect of the invention may have various structures generally employed in printed wiring boards.
Actions of the second aspect of the present invention will be described below.
In the printed wiring board according to the second aspect of the invention, a glass cloth-reinforced prepreg constitutes the internal insulating layer, while a resin constitutes the external insulating layer. That is, since the internal insulating layer contains the glass cloth, coefficient of water absorption can be reduced in the layer. Accordingly, the coefficient of water absorption of the internal insulating layer as a whole can be reduced.
Therefore, the absolute amount of water to be contained in the internal insulating layer is reduced, in turn, the absolute amount of water vapor to be formed by vaporization of the water content is reduced. Thus, the amount of water vapor collecting between the layers is reduced, increasing interlayer adhesion.
That is, the printed wiring board according to the second aspect of the invention has a highly reliable structure, since it hardly undergoes interlayer delamination.
Further, since the external insulating layer is of a resin, it facilitates formation of fine patterns. Therefore, the printed wiring board according to the second aspect of the invention facilitates formation of a high-density substrate.
As described above, according to the second aspect of the invention, printed wiring boards which hardly undergo interlayer delamination and can maintain high reliability even if the printed wiring board is allowed to have a higher multilayer structure, can be provided.
Further, the printed wiring board according to the second aspect of the invention can be utilized, for example, as memory modules, multichip modules, mother boards, daughter boards and plastic packages.
It is preferred to form two or more internal insulating layers. According to this structure, printed wiring boards having higher multilayer structures and high reliability can be obtained.
The coefficient of water absorption in the internal insulating layer is preferably 0.1 to 0.3%. Thus, the effects to be brought about according to the second aspect of the invention can be secured. It is difficult to form such prepregs as having coefficients of water absorption of less than 0.1%; whereas prepregs having coefficients of water absorption of more than 0.3% contain too much water to exhibit the effect to be brought about according to the second aspect of the invention.
A third aspect of the invention is a method of manufacturing a printed wiring board having a plurality of conductive layers which are built up via insulating layers respectively and are electrical connected to one another via interconnecting through holes. The method comprises the steps of forming conductive layers on a plurality of insulating layers respectively; laminating and press-bonding the resulting insulating layers to form a multilayer substrate; irradiating a laser beam upon the multilayer substrate at interconnecting through hole-forming portions to define interconnecting through holes such that the bottoms of these through holes reach the conductive layers; fusing solder balls against the interconnecting through holes and filling them with the solder.
Actions and effects of the third aspect of the invention will be described.
In the third aspect of the invention, after the insulating layers are laminated, a laser beam is irradiated to form interconnecting through holes. Accordingly, interconnecting through holes penetrating all of the insulating layers are formed by a single hole-defining procedure. Further, there is no need of forming interconnecting through hole-defining through holes in the respective insulating layers independently, facilitating formation of interconnecting through holes.
Furthermore, according to the third aspect of the invention, interconnecting through holes having different depths can be formed by the single hole-defining procedure.
Unlike the prior art, insulating layers need not be positioned for securing continuity of the through holes. Further, even small interconnecting through holes can be formed accurately.
Further, the interconnecting through holes are filled with a solder, and solder balls are fused to the openings of the interconnecting through holes, so that electric currents flowing across the internal conductive layers can be taken out easily through the solder and solder balls.
The walls of the interconnecting through holes are preferably covered with metal plating films, and thus conductivity can be imparted to these through holes.
The conductive layers preferably have a thickness of 10 to 70 xcexcm. If they have a thickness of less than 10 xcexcm, holes are likely to be formed in the conductive layers by the laser beam irradiation; whereas if they have a thickness of more than 70 xcexcm, patterning of the conductive layers is likely to be difficult.
The insulating layers are preferably flexible films made of a glass fiber-reinforced resin. Such insulating layers facilitate the hole-defining procedures using laser beam, and besides thinning of printed wiring boards can be realized.
As the laser beam 341, for example, a CO2 laser and an eximer laser can be used.
As the insulating layer, for example, synthetic resin single substances, resin base materials containing synthetic resins and inorganic fillers, cloth base materials containing synthetic resins and inorganic cloth, etc. can be used. The synthetic resins include, for example, epoxy resins, phenol resins, polyimide resins, polybutadiene resins and fluororesins. Insulating layers formed using such synthetic resins only are occasionally laminated as prepregs or solder resists between other insulating layers.
Further, the inorganic fillers to be added to the synthetic resins include, for example, glass short fibers, silica powders, mica powders, alumina and carbon. Base materials containing mixtures of synthetic resins and inorganic fillers show high strength compared with those made of synthetic resin single substances.
Meanwhile, the cloth base materials referred to above mean those substrates made of woven or knitted fabric cloth and synthetic resins such as glass-epoxy substrates and glass-polyimide substrates. Such cloth base materials include those obtained by impregnating the cloth with synthetic resins. Further, materials of the cloth include glass-fiber cloth, carbon cloth, aramid cloth, etc. As the synthetic resins those as described above are employed.
The conductive layers referred to above mean conductive patterns which are formed parallel to the surfaces of insulating layers, for example, wiring patterns, pads, lands and terminals. The conductive patterns are formed, for example, by etching metal foils or by metal plating.
The printed wiring board manufactured according to the third aspect of the invention can be utilized, for example, as memory modules, multichip modules, mother boards, daughter boards and plastic packages.
A fourth aspect of the invention is a printed wiring board comprising an interconnecting through hole penetrating an insulating substrate, a covering pad covering one opening of the interconnecting through hole, and a conductor circuit provided along the peripheral edge of the other opening which remains open; wherein the covering pad and the conductor circuit are electrically connected to each other via a metal plating film covering the wall of the interconnecting through hole; and a solder ball for external connection is bonded onto the surface of the covering pad.
Actions and effects of the fourth aspect of the invention will be described.
In the fourth aspect of the invention, one opening of each interconnecting through hole is covered with a covering pad on which a solder ball is bonded. Accordingly, the covering pad for bonding a solder ball can be located substantially in alignment with the interconnecting through hole.
Therefore, the area occupied by the interconnecting through hole coincides with the area occupied for bonding the solder ball, so that there is no need of securing the area for forming interconnecting through holes and the area for bonding solder balls separately, thus achieving high-density packaging of interconnecting through holes and solder balls.
Further, since the areas to be occupied by the interconnecting through holes and solder balls are narrowed to afford extra spaces on the surface of the insulating substrate, conductor circuits and the like can be formed on such extra spaces, enabling high densification of surface packaging on the insulating substrate. Besides, the fourth aspect of the invention fully satisfies the requirements particularly for multilayer build-up type printed wiring boards which require high-density surface packaging.
The solder balls are preferably located in alignment with the central axes of the interconnecting through holes respectively. Since the interconnecting through holes and the solder balls can be aligned respectively, the areas to be occupied by both of them can further be narrowed.
The solder balls may be located at positions offset from the interconnecting through holes respectively. In this case, larger areas are required for bonding solder balls and for forming the interconnecting through holes compared with the case where they are aligned. However, they can be located in small areas compared with the prior art where they are located completely separately.
It is preferred that the surface of the insulating substrate is covered with a solder resist, and also the interconnecting through holes are filled with the solder resist. Thus, the conductor circuit formed on the surface of the insulating substrate and the metal plating films formed on the walls of the interconnecting through holes can be protected from moisture and flawing. The solder ball-connecting portions are not covered with the solder resist but are exposed. In the case where terminal connecting portions for terminals other than solder balls are to be secured, such portions are not covered with the resist but are exposed. The interconnecting through holes may be filled with a filler of conductive materials such as a solder in place of the solder resist.
A fifth aspect of the invention is a printed wiring board comprising an interconnecting through hole penetrating an insulating substrate, an annular pad disposed along the peripheral edge of one opening of the interconnecting through hole so as not to cover the opening, a covering pad covering the other opening of the interconnecting through hole and a conductor circuit connected to the covering pad; wherein the annular pad and the covering pad are electrically connected to each other by a metal plating film covering the wall of the interconnecting through hole; and a solder ball for external connection is bonded onto the surface of the annular pad.
In the fifth embodiment of the invention, an annular pad is located along the peripheral edge of one opening of each interconnecting through hole, and a solder ball is bonded onto the surface of the pad. Accordingly, the solder ball can be located substantially in alignment with the interconnecting through hole. Therefore, the area to be occupied by the interconnecting through hole coincides with the area to be occupied for bonding the solder ball, so that there is no need of securing the area for forming interconnecting through holes and the area for bonding solder balls separately, thus achieving high-density packaging of interconnecting through holes and solder balls.
Further, since the areas to be occupied by the interconnecting through holes and solder balls are narrowed to afford extra spaces on the surface of the insulating substrate, conductor circuits and the like can be formed on such extra spaces, enabling high densification of surface packaging on the insulating substrate.
It is preferred that the solder balls are located in alignment with the central axes of the interconnecting through holes respectively and that each interconnecting through hole is filled with the solder as a lower part of the solder ball. Since the interconnecting through holes and the solder balls can be aligned respectively as described above, the areas to be occupied by both of them can further be narrowed.
The solder balls may be located at positions offset from the interconnecting through holes respectively. In this case, larger areas are required for bonding solder balls and for forming the interconnecting through holes compared with the case where they are aligned. However, they can be located in small areas compared with the prior art where they are located completely separately.
The surface of the insulating substrate is preferably covered with a solder resist. Thus, the conductor circuit formed on the surface of the insulating substrate can be protected from moisture, flawing, etc. The solder ball-connecting portions on the covering pads are not covered with the solder resist but are exposed. In the case where terminal connecting portions for terminals other than solder balls are to be secured, such portions are not covered with the resist but are exposed.